As a contract for using a communication terminal, there is a prepaid type contract which enables to use the communication terminal according to a predetermined charge which has been prepaid. In the case of the prepaid type contract, an available usage time of the communication terminal is limited. Then, in the case that an expiration time of the available usage time becomes close, a user can recognize that the expiration time of the available usage time becomes close if the user receives notification that the expiration time becomes close.
In the case of a 3G (3rd Generation) service, a mechanism that, just before an available usage time of a communication terminal which a prepaid user uses expires, a predetermined alert tone is sent to the communication terminal is standardized (refer to NPL 1).
On the other hand, architecture for realizing a CAMEL (Customized Applications for Mobile network Enhanced Logic) prepaid service in IMS (IP Multimedia Subsystem) is defined on the basis of the specification of 3 GPP (3rd Generation Partnership Project) (refer to NPL 2).